Inverters have been installed in many types of electrical equipment, as an efficient variable-speed control unit. Inverters are switched at a frequency of several kHz to tens of kHz, to cause a surge voltage at every pulse thereof. Inverter surge is a phenomenon in which reflection occurs at a breakpoint of impedance, for example, at a starting end, a termination end, or the like of a connected wire in the propagation system, and as a result, a voltage up to twice as high as the inverter output voltage is applied. In particular, an output pulse occurred due to a high-speed switching device, such as an IGBT, is high in steep voltage rise. Accordingly, even if a connection cable is short, the surge voltage is high, and further voltage decay due to the connection cable is low. As a result, a voltage almost twice as high as the inverter output voltage occurs.
As coils for electrical equipment such as inverter-related equipment, for example, high-speed switching devices, inverter motors and transformers, insulated wires, which are enameled wires, are mainly used as magnet wires in the coils. Accordingly, as described above, since a voltage nearly twice as high as the inverter output voltage is applied in inverter-related equipment, it has been required in insulated wires to minimize partial discharge deterioration, which is attributable to inverter surge.
In general, partial discharge deterioration means a phenomenon in which the following deteriorations of the electrical insulating material occur in a complicated manner: molecular chain breakage deterioration caused by collision with charged particles that have been generated by partial discharge (discharge at a portion in which fine void defect exists); sputtering deterioration; thermal fusion or thermal decomposition deterioration caused by local temperature rise; and chemical deterioration caused by ozone generated due to discharge, and the like. The electrical insulating materials which actually have been deteriorated by partial discharge show reduction in the thickness.
In order to prevent deterioration of an insulated wire caused by such partial discharge, insulated wires having improved resistance to corona discharge by incorporating particles into an insulating film have been proposed. For example, an insulated wire incorporating metal oxide fine particles or silicon oxide fine particles into an insulating film (see Patent Literature 1), and an insulated wire incorporating silica into an insulating film (see Patent Literature 2) have been proposed. These insulated wires reduce erosive deterioration caused by corona discharge, by the insulating films containing particles. However, the insulated wires having insulating films containing these particles have problems that the effect is insufficient so that a partial discharge inception voltage is decreased and flexibility of the coated film is decreased.
There is also available a method of obtaining an insulated wire which does not cause partial discharge, that is, an insulated wire having a high partial voltage at which partial discharge occurs. In this regard, a method of making the thickness of the insulating layer of an insulated wire thicker, or using a resin having a low relative dielectric constant in the insulating layer can be considered.
However, when the thickness of the insulating layer is increased, the resultant insulated wire becomes thicker, and as a result, size enlargement of electrical equipment is brought about. This goes against the demand in recent miniaturization of electrical equipment represented by motors and transformers. For example, specifically, it is no exaggeration to say that the performance of a rotator, such as a motor, is determined by how many wires are held in a stator slot. As a result, it has been required in recent years to particularly increase the ratio (space factor) of the sectional area of conductors to the sectional area of the stator slot. Therefore, increasing the thickness of the insulating layer leads to a decrease in the space factor, and this is not desirable when the required performance is taken into consideration.
On the other hand, with respect to the relative dielectric constant of an insulating layer, most of the resins that are generally used as a material for the insulating layer have a relative dielectric constant from 3 to 4, and thus there is no resin having a specifically low relative dielectric constant. Furthermore, in practice, a resin having a low relative dielectric constant cannot always be selected necessarily when other properties that are required for the insulating layer (heat resistance, solvent resistance, flexibility and the like) are taken into consideration.
As a means for decreasing a substantial relative dielectric constant of the insulating layer, such a measure has been studied as forming the insulating layer from foam, and foamed wires containing a conductor and a foamed insulating layer have been widely used as communication wires. Conventionally, foamed wires obtained by, for example, foaming an olefin-based resin such as polyethylene or a fluorine resin have been well-known. Specific examples include foamed polyethylene insulated wires (see Patent Literature 3), foamed fluorine resin insulated wires (see Patent Literature 4), and the like.
However, these conventional foamed wires have a poor scratch resistance and therefore cannot satisfy properties required for the insulated wire.